The present invention relates to a semiconductor pressure sensor making use of the piezoresistive effect and a method of fabricating the same.
By making use of the fact that a resistance is varied due to the piezoresistive effect by applying a mechanical stress, there is used a semiconductor pressure sensor for detecting the change in the resistance due to piezoresistive effect to measure the pressure by thinning a portion of a single crystal silicon substrate to form a diaphragm, by forming a strain gauge of a diffusion layer in an epitaxial layer formed in the diaphragm and by deforming the strain gauge under pressure.
The manuscripts of the 6th "THE BASIC AND APPLICATION OF SENSOR" symposium, P 27-28, entitled "Micro-Diaphragm Pressure Sensor" disclose a semiconductor pressure sensor which will be described with reference to FIG. 20. That device includes a substrate 100 having a crystal plane (100), which has an etched recess or cavity 100a. A silicon oxide film 101 is formed in a predetermined region over the main surface of substrate 100 A polycrystalline silicon layer 102 is formed in a predetermined region in the portion, which is not formed with the silicon oxide film 101 (but is formed with the cavity 100a), and over the silicon oxide film around that portion and which is removed by etching. A first silicon nitride film 103 is formed over the polycrystalline silicon layer 102 and the silicon oxide film 101 and has an etch-hole 106 therein. Over this first silicon nitride film 103, there are formed a strain gauge 104 made of polycrystalline silicon of a predetermined pattern and a second silicon nitride film 105. An undercut-etching is accomplished through the etch-hole 106 to form the cavity 100a in a desired position by making use of the relatively high etching rate of the polycrystalline silicon. The silicon nitride film overlying the cavity 100a is used as a diaphragm. Finally, the pressure sensor is constructed by sealing the etch-hole 105 with a third silicon nitride film 107 which is formed by the CVD (i.e., Chemical Vapor Deposition).
In the semiconductor pressure sensor thus constructed, the single crystal silicon substrate 100 is etched from its surface at the side to be formed with the stain gauge 104, to form the cavity 100a and use the silicon nitride film thereover as the diaphragm. This makes it possible to reduce the volume of the single crystal silicon substrate 100 to a relatively small value and to thin the diaphragm thereby to reduce its size. Since, however, the strain gauge 104 is made of the polycrystalline silicon, the semiconductor pressure sensor has a lower sensitivity than that made of single crystal silicon so that its characteristics are not very uniform. In this connection, there has been proposed a technique in which a single crystal strain gauge is formed by recrystallizing the polycrystalline silicon. According to this technique, however, it is difficult to make the characteristic more uniform, and the recrystallization raises the production cost.
Since, moreover, the diaphragm is not smooth in the portion in which the etch-hole 103 is formed, that portion is weak against a mechanical strain. When the etch-hole 106 is to be sealed with the third silicon nitride film 107, on the other hand, the dispersions are liable to cause the third silicon nitride film 107 to bury the etch-hole 106 so that the pressure sensor's output characteristics are unstable.